The present invention relates to magnetic gear arrangements, particularly magnetic gear arrangements having a variable gear ratio.
Gearboxes and gear arrangements are utilised in a wide range of situations in order to couple drive mechanisms. Traditionally, gearboxes have been formed from gear wheels having appropriate teeth numbers and sizes to provide a desired gear ratio. However, such gearboxes have a number of disadvantages. Firstly, they require the use of lubricating oils, which may act as contaminants or fire hazards and may prove ineffective in hot or cold environments, where the oil viscosity varies, or in a low pressure environment, where the oil may evaporate. Furthermore, gearboxes based on gear wheels may be noisy, making them unacceptable for low noise environments such as in hospitals, libraries and residential areas, or for clandestine military activities.
More recently, magnetic gearboxes have been provided which comprise respective gear rotors with interpoles between them. The rotors incorporate permanent magnets, and the interpoles act to modulate the magnetic flux transferred between the gear rotors. Such magnetic gearboxes enable a speed-changing mechanical device to be provided in which there is no mechanical contact between input and output shafts, thus avoiding many of the problems of noise and wear that arise in gearboxes having contacting moving parts.
FIG. 1 shows a schematic cross-sectional view of a magnetic gear arrangement of the prior art. The magnetic gear arrangement 1 is an epicyclic gearbox and comprises an inner rotor 2 and an outer rotor 6. Permanent magnets providing respective pole pairs 4, 8 are fixed to the inner and outer rotors 2, 6, the opposing poles of each permanent magnet being respectively indicated by dark and light shading. The permanent magnets 4 affixed to the inner rotor 2 have alternating polarity along the circumference of the rotor. Similarly, the permanent magnets 8 affixed to the outer rotor 6 have alternating polarity along the circumference of that rotor. Typically, one rotor is mechanically coupled to a drive mechanism and the other rotor is mechanically coupled to a driven mechanism.
The inner and outer rotors 2, 6 have different numbers of pole pairs 4, 8. Typically, the number of pole pairs of the outer rotor 6 is greater than the number of pole pairs of the inner rotor 2.
Pole elements 10 are provided between the inner rotor 202 and the outer rotor 6 and form an array to provide a coupling element having a cylindrical shape.
Each pole element 10 forms one interpole for modulating the magnetic fields produced by the inner rotor 2 and the outer rotor 6, so as to couple the two fields and hence the motion of the rotors. The angular position of the interpoles is a factor in determining the gearing of the magnetic gearbox.
For example, the motion of the rotors 2, 6 may be either co-rotational or counter-rotational, depending on the number of magnets affixed to each rotor and the angular position/number of interpoles. Further, the co-rotational and counter-rotational modes have respective optimum interpole angular spacings which maximise the effectiveness of coupling between the rotors for that mode and determine the gear ratio between the rotors. By varying the angular position of the interpoles it possible to provide different gear ratios between the rotors, for example by using harmonics of the magnetic fields produced by the inner rotor and/or the outer rotor, but at a cost of less effective coupling. Further, the number of such ratios is limited by the fixed number of poles on the rotors.
WO 2007/135369 discusses a variety of magnetic gear arrangements.
It would be desirable to provide gearboxes in which the gear ratio is readily adjustable, but without the loss of coupling effectiveness associated with for example interpole arrangements that make use of magnetic field harmonics.